A first example of a prior art foldable communication terminal is one described in Patent Document 1. The prior art foldable communication terminal is described below referring to drawings. FIG. 20 shows the structure of a prior art communication terminal and the principle of operation of the antenna provided thereon. FIG. 20A shows a state where the casings are closed and FIG. 20B a state where the casings are opened. In FIGS. 20A and 20B, the communication terminal includes a first casing 202 connected to a telescopic antenna 201 and a second casing 203 positioned on the side of the antenna, the second casing including a metal material or a material including a metal, the first and second casings 202, 203 interconnected. The communication terminal is designed so that the high-frequency current flowing through the first casing 202 where the antenna 201 is equipped will also flow into the second casing 203. The directions of the currents are shown by arrows G, H, I and J.
A second example of a prior art foldable communication terminal is disclosed in Patent Document 2. This communication terminal has a structure that a first casing where an antenna is provided at one end is coupled to a second casing in a foldable manner via a hinge and a flexible board wound in a helical fashion is arranged in the hinge. FIG. 21 shows a change of the flexible board which accompanies opening/closing operation of the casing in case the flexible board is provided in a helical fashion. In FIG. 21, (a) shows a state where the casings are opened and (b) shows a state where the casings are closed. The shape of the flexible board 204 in FIG. 21(a) while the casing is opened is shown by solid lines, and by chain double-dashed lines while the casing is closed so as not to change the angle of twist.
(Patent Document 1)
    JP-A-2002-33804(Patent Document 2)    JP-A-6-311216
On the communication terminal of the first prior art example, the first and second casings are connected so that the currents flowing through the casings 202, 203 will be in same phase with each other (arrows G and H are in the same direction). That is, each current flows from the end where the antenna 201 is provided to the opposite end. When the two casings are opened, the casing current (arrow I) flowing through the first casing 202 stays the same as that the casing current (arrow G) flowing when the casing is closed. On the other hand, the direction of the casing current (arrow J) flowing through the second casing 203 is different from, or in opposite phase with, that of the casing current (arrow H) flowing through the first casing member 202. Thus, the radiation effect could be deteriorated when the state of the casings is shifted from the closed state to the opened state. Depending on the connection manner of casing and an antenna, the radiation effect could be deteriorated.
The communication terminal in the second example has a structure that the antenna protrudes from the casing. Such a protruding antenna has a problem that the antenna gets snagged on the edge of a pocket when the user takes out the communication terminal from the pocket. In recent years, a foldable hinged communication terminal with an integrated antenna has been in growing demand.
As shown in FIG. 21, the flexible board 204 wound in a helical manner is subject to a bending stress and a twisting stress with the opening/closing operation of the casings, which could break the conductive portion (circuit pattern) of the flexible board 204. In order not to change the twist angle of the flexible board wound clockwise from the bottom to the top, the flexible board is wound in chain double-dashed lines shown in FIG. 21(a), thus avoiding the twisting stress. However, both ends of the flexible board 204 are fixed to the same positions of the casings (in the same horizontal direction) irrespective of the opening/closing operation of the casings. When the casing is closed into a shape shown in the solid lines in FIG. 21(b) from the open state, the twist angle changes to an approximate P/2. Thus, the flexible board 204 is subject to a bending stress and a twisting stress in accordance with the opening/closing operation of the casings.
In general, a flexible board is flexible so that it is resistant enough to a bending stress. A composite stress exerted on the flexible board invites a wire break or rupture. Thus, in the prior art, repeating the opening/closing operation of casings could bring about the wire break or rupture of the casings, which has required improvement.
The invention has been accomplished in view of the above problems and has as an object to provide a communication terminal capable of suppressing the degradation of an antenna gain irrespective of whether the casing of the communication terminal is opened state or closed state.